1. Field of the Invention
The present invention relates to an alarm detecting system suitable for use in a redundancy configuration circuit, for effecting the action of switching between respective redundantly-configured circuits employed in radio equipment or the like.
2. Description of the Related Art
FIG. 1 is a block diagram showing the structure of an alarm detecting system suitable for use in a conventional redundancy configuration circuit. In the same drawing, reference numerals 40a, 40b respectively indicate receiving circuits which receive a signal supplied from an input terminal 11 and are set up in redundant configurations. Reference numerals 2a, 2b respectively indicate alarm detecting circuits of the receiving circuits 40a, 40b. Further, designated by numeral 3 is a system-in-use/backup-system specifying circuit for outputting specifying signals 13a, 13b therefrom in response to alarm detection signals 12a, 12b supplied from the alarm detecting circuits 2a, 2b, respectively.
The operation of the alarm detecting system will now be described below. Let's now assume that the receiving circuit 40a is of a system in use and the remaining receiving circuit 40b is of a backup system. When a failure or malfunction occurs in the receiving circuit 40a, the alarm detecting circuit 2a detects it and brings the alarm detection signal 12a into a significant condition (alarm detected condition). It is needless to say that the receiving circuit 40b does not detect the malfunction and bring the alarm detection signal 12b into the significant condition.
That is, the system-in-use/backup-system specifying circuit 3 senses only the significant condition of the alarm detection signal 12a. As a result, the system-in-use/backup-system specifying circuit 3 recognizes that the receiving circuit 40a which is of the system in use, is in an improper state. Based on the result of the recognition, the system-in-use/backup-system specifying circuit 3 then outputs the specifying signal 13a so as to bring the receiving circuit 40a to the backup system and outputs the specifying signal 13b so as to bring the receiving circuit 40b serving as the backup system to the system in use. The receiving circuits 40a, 40b respectively effect a changeover from a function of the system in use to a function of the backup system or vise versa in response to the specifying signals 13a, 13b.
When the application of a signal to the input terminal 11 is stopped owing to external factors, the alarm detecting circuits 2a, 2b respectively detect that the application of the signal thereto has been stopped and bring the alarm detection signals 12a 12b into the significant condition. When the system-in-use/backup-system specifying circuit 3 detects the two alarm detection signals, it recognizes that a failure or malfunction has been produced by the external factors, and does not output the specifying signals 13a, 13b. That is, the action of switching between the system in use and the backup system is not effected.
Incidentally, there is known a technique disclosed in Japanese Patent Application Laid-Open Publication No. 2-226935 as a prior art.
The alarm detecting system for the conventional redundancy configuration circuit is constructed as described above. Thus, when the two alarm detection signals 12a, 12b are brought into the significant condition owing to the external factors, the difference is produced between the timing for outputting the alarm detection signal 12a and the timing for outputting the alarm detection signal 12b according to the difference in detection time between the two alarm detecting circuits 2a and 2b. When the alarm detection signal 12a of the system in use is first brought to the significant condition, the system in use is sometimes switched over to the backup system when unnecessary.